Lead frame material

ABSTRACT

A lead frame material, which can be provided with an etching stop layer without requiring a troublesome process such as vapor deposition and has an excellent heat-resistance, is obtained by forming a nickel-phosphorus alloy layer (B) of 1.6 to 10 μm thickness containing 0.3 to 1.0 wt % of phosphorus on a copper or copper alloy layer (A) of 35 to 300 μm thickness, and forming an optional copper layer (C) of 0.2 to 30 μm thickness on the nickel-phosphorus alloy layer (B).

TECHENICAL FIELD

The present invention relates to a lead frame material useful for theproduction of lead frames, such as those for semiconductor devices.

BACKGROUND ART

As the integration of IC's and LSI's is increasing, the requiredpin-count is increasing, necessitating fine patterning of leads. Etchingis suited to the production of lead frames having leads of fine patternsfrom lead frame materials. For example, Japanese Patent ApplicationUnexamined Publication No. 3-148856 (1991) discloses production of leadframes by etching a three-layered lead frame material comprising twometal layers of different thicknesses and an aluminum etching stop layersandwiched therebetween.

In the method, the etching stop layer enables selective etching of oneof the two metal layers of different thicknesses. The thicker metallayer is to be the base material of the lead frame and is thick enoughto afford the required mechanical strength. The thinner metal layer isto be the bonding areas for connection with IC's and is thin enough toform finely patterned bonding areas. Thus lead frames having both finebonding areas for connection with IC's and sufficient mechanicalstrength can be produced by the selective etching.

The etching stop layer of the three-layered lead frame material used inthe method, however, is an aluminum layer formed by vapor deposition.The vapor deposition for forming the aluminum layer disadvantageouslyrequires a complicated process, which increases the production cost ofthe lead frame material.

Further, in the course of producing lead frame by the above-describedmethod, the three-layered lead frame material is exposed to hightemperatures when, for example, polyimide films are bonded as aninsulating protective films to the surfaces of the lead areas withpolyamic acid adhesives by curing the adhesives at 350° C. or higher.The etching stop layer therefore requires a good heat-resistance enoughto withstand such a high temperature. For example, nickel layers, whichare readily formed by electroplating, cannot act as etching stop layers,because copper diffuses from metal layers into the nickel layers at hightemperatures of 350° C. or higher.

In Japanese Patent Application Unexamined Publication No. 5-121617(1993) is disclosed the production of lead frames from a two-layeredlead frame material comprising a first copper layer of 80 to 150 μmthickness and a second aluminum layer of 10 to 50 μm thickness, byselectively plating a third copper layer on the second aluminum layer,and then patterning the first copper layer to form inner leads. Havingthe aluminum layer as an etching stop layer, the two-layered lead framematerial used in the method also involves the same problem as that withthe three-layered lead frame material.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a three-layered leadframe material and a two-layered lead frame material, which have anetching stop layer being able to formed readily by electroplatingwithout requiring a troublesome process such as vapor deposition andhaving excellent heat-resistance.

As the results of studies to solve the above-described problems, theinventors have found that a specific alloy layer of a specific thicknessis useful as the etching stop layer of three-layered and two-layeredlead frame materials because it can be formed readily by electroplatingwithout requiring a troublesome process such as vapor deposition andcontributes excellent heat-resistance to the three-layered andtwo-layered lead frame materials. They have completed the presentinvention on the basis of the finding.

That is, the present invention provides a lead frame material,comprising

a copper or copper alloy layer (A) of 35 to 300 μm thickness;

a nickel-phosphorus alloy layer (B) of 1.6 to 10 μm thickness which isformed on the copper or copper alloy layer (A) and contains 0.3 to 1.0wt % of phosphorus; and

a copper layer (C) of 0.2 to 30 μm thickness which is formed on thenickel-phosphorus layer (B).

The present invention further provides a lead frame material, comprising

a copper or copper alloy layer (A) of 35 to 300 μm thickness; and

a nickel-phosphorus alloy layer (B) of 1.6 to 10 μm thickness which isformed on the copper or copper alloy layer (A) and contains 0.3 to 1.0wt % of phosphorus.

BEST MODE FOR CARRYING OUT THE INVENTION

The 35 to 300 μm thick copper or copper alloy layer (A) of the leadframe materials of the present invention is to be patterned into theouter leads or the like of lead frames. If it is less than 35 μm thick,the mechanical strength of the outer leads will be insufficient, and ifmore than 300 μm thick, it will take a long time to etch and lower theproductivity. It is preferably 50 to 200 μm thick. Preferred copperalloys include alloys of copper with at least one metal selected fromSn, Ni, Zn, P, Fe, Zr, Cr, Mg and Si, and the content of other metalsthan copper in the copper alloy is preferably 0.01 to 5 wt %. The copperor copper alloy layer (A) preferably has a surface roughness Ra of 0.1to 2.0 μm, more preferably 0.2 to 0.8 μm. If the surface roughness Ra isless than 0.1 μm, the adhesion of the nickel-phosphorus alloy layer (B)may be low, and if more than 2 μm, pin-holes may be made in thenickel-phosphorus alloy layer (B).

The 0.2 to 30 μm thick copper layer (C) of the three-layered lead framematerial is to be patterned into fine patterns. If it is less than 0.2μm or more than 30 μm in thickness, fine patterns cannot be formed. Itis preferably 0.5 to 10 μm thick.

The 1.6 to 10 μm thick nickel-phosphorus alloy layer (B) is not etchablewith the etchant for the copper or copper alloy layer (A). The functionof this layer in the three-layered lead frame material is to prevent oneof the layers sandwiching it from being etched while the other is beingetched. Compared with the conventional aluminum layer which has beenformed by vapor deposition, the nickel-phosphorus alloy layer (B) isadvantageous because it can be formed readily by electroplating withoutrequiring the troublesome process, thereby lowering the production costof the lead frame materials. Further, it has good heat-resistancebecause the phosphorus contained in the nickel-phosphorus alloy inhibitsthe diffusion of copper from the adjacent layers into thenickel-phosphorus alloy layer (B) even at high temperatures. Thephosphorus content in the nickel-phosphorus alloy layer (B) is 0.3 to1.0 wt %, preferably 0.5 to 0.8 wt %. If the phosphorus content is lessthan 0.3 wt %, the heat-resistance will be too low to prevent thediffusion of copper into the nickel-phosphorus alloy layer (B), and thelayer cannot work as an etching stop layer. If it is more than 1.0 wt %,the productivity will be low due to the low nickel-phosphoruselectrodeposition efficiency.

If the nickel-phosphorus alloy layer (B) is less than 1.6 μm thick, itcannot fulfil the function of an etching stopping layer, namely thefunction of protecting one of the adjacent metal layers while the otheris being etched. If it is more than 10 μm thick, the removal of theetching stop layer in the final stage of the production of lead frameswill take a long time, thereby lowering the productivity. Preferredthickness is 2.0 to 5.0 μm.

The surface roughness Ra of the nickel-phosphorus alloy layer (B)somewhat depends on the surface roughness of the underlying copper orcopper alloy layer (A), but is preferably 0.1 to 0.8 μm.

For the purpose of anticorrosion, the surface of the lead frame materialis preferably treated with a chromate or a chromate containing zinccompounds.

The lead frame material of the present invention may be produced byforming the 1.6 to 10 μm thick nickel-phosphorus alloy layer (B) on oneside of the 35 to 300 μm thick copper or copper alloy layer (A) bynickel-phosphorus plating, and, in case of the three-layered lead framematerial, additionally forming the 0.2 to 30 μm thick copper layer (C)on the surface of the nickel-phosphorus alloy layer (B) by copperplating. If necessary, a chromate layer may be formed on the exposedsurface of the copper layer (C).

When the two-layered lead frame material is used, fine bonding areas forconnection with IC's can be formed by forming plating resist patterns onthe nickel-phosphorus alloy layer (B) and then plating copper thereon.When the three-layered lead frame material is used, fine bonding areasfor connection with IC's can be formed by etching the copper layer (C).

The copper-phosphorus plating is carried out preferably by using thefollowing plating solution under the following plating conditions.

(1) The composition of plating bath

    ______________________________________                                        nickel sulfate         200-300  g/l                                             boric acid 10-100 g/l                                                         phosphorous acid 0.2-20 g/l                                                   sodium o-sulfobenzoic acid imide 1-50 g/l                                     magnesium sulfate 10-200 g/l                                                ______________________________________                                    

(2) Electrolysis conditions

pH: 1.6-3.0, current density: 1-10 A/dm²,

solution temperature: 20-70° C.

Preferred examples of the semiconductor devices which may be producedusing the lead frame materials of the present invention include TBGA(tape ball grid array) devices and CSP (chip size package) devices.

The present invention will be described in more detail with reference tothe following Examples. These examples, however, are not to be construedto limit the scope of the invention.

EXAMPLE 1

A 150 μm thick copper alloy foil (produced by Mitsubishi Shindo Co.,Ltd., copper-nickel-tin alloy, trade name: TAMAC15, surface roughnessRa: 0.2 μm) was prepared, and was coated on its non-plating surface witha plastic film for preventing the deposition of electrolytic metal.

The following steps (1-5) were carried out in order by using the foil,to produce a three-layered lead frame material by forming on the copperalloy layer a nickel-phosphorus alloy layer (thickness: 2 μm, phosphoruscontent: 0.8 wt %, surface roughness Ra: 0.2 μm) and a copper layer(thickness: 2 μm, surface roughness Ra: 0.2 μm). After each of the steps1-5, washing with water was carried out.

Production Steps

1. Degreasing

(1) Degreasing solution

sodium orthosilicate: 30 g/l

sodium carbonate: 20 g/l

sodium hydroxide: 20 g/l

(2) Electrolytic degreasing conditions

current density: 5 A/cm², treating time: 30 seconds,

solution temperature: 40° C.

cathode: copper alloy foil, anode: iridium oxide

2. Washing with acid

(1) Washing solution

sulfuric acid: 25 g/l

(2) Washing conditions

treating time: 30 seconds, solution temperature: 20° C.

3. Production of an electrolytic nickel-phosphorus alloy layer

(1) Plating solution

    ______________________________________                                        nickel sulfate          300    g/l                                              boric acid 40 g/l                                                             phosphorous acid 4 g/l                                                        sodium o-sulfobenzoic acid imide 10 g/l                                       magnesium sulfate 80 g/l                                                    ______________________________________                                    

(2) Electrolysis conditions

current density: 1.1 A/dm²

electrolysis time: 10 minutes

solution temperature: 35° C.

pH: 2.5

anode: iridium oxide

4. Production of an electrolytic copper layer

(1) Plating solution

    ______________________________________                                        copper sulfate       280    g/l                                                 sulfuric acid 70 g/l                                                          additive (gelatin) 3 ppm                                                      additive (Cl.sup.-) 10 ppm                                                  ______________________________________                                    

(2) Electrolysis conditions

current density: 3.5 A/cm²

electrolysis time: 4 minutes

solution temperature: 35° C.

anode: iridium oxide

5. Production of an anticorrosion layer

(1) Treating solution

    ______________________________________                                        sodium bichromate      3.5   g/l                                              ______________________________________                                    

(2) Treating conditions

dipping time: 25 seconds

solution temperature: 20° C.

pH: 4.7

6. Drying

drying conditions: temperature: 100° C., time: 5 minutes

The three-layered lead frame material produced through theabove-described steps was heated (200° C., 300° C., 400° C., 500° C.)for 30 minutes in the atmosphere of gaseous N₂, and then the followinghigh-temperature thermomigration test was carried out to evaluate theheat-resistance of the lead frame material.

High-temperature Thermomigration Test

The test pieces heated to the different temperatures were analyzed ontheir sections (crossing the interface between the nickel-phosphorusalloy layer and the thinner copper layer) with an SEM (scanning electronmicroscope) and an Auger electron spectroscopy analyzer, to obtain thedegree of the thermomigration alloying of metals caused by heating asthe thermomigration percentages (%), which are given in Table 1. Thelesser thermomigration percentage (%) indicates the lesser alloying ofmetals, and indicates the excellence of the lead frame material inheat-resistance and the ability of selective etching.

Comparative Example 1

A lead frame material was produced in the same manner as in Example 1using the same copper alloy foil as that used in Example 1, except thatthe step 3 was replaced by the following step 3' for production of anelectrolytic nickel layer to replace the nickel-phosphorus alloy layerby a nickel layer (thickness: 2 μm, surface roughness Ra: 0.2 μm).Thermomigration test was carried out in the same manner as in Example 1,and the results are given in Table 1.

3'. Production of an electrolytic nickel layer

(1) Plating solution

    ______________________________________                                        nickel sulfate         280      g/l                                             boric acid 40 g/l                                                             sodium o-sulfobenzoic acid imide 5 g/l                                      ______________________________________                                    

(2) Electrolysis conditions

current density: 1.0 A/dm²

electrolysis time: 10 minutes

solution temperature: 35° C.

pH: 2.5

anode: iridium oxide

                  TABLE 1                                                         ______________________________________                                                 Thermomigration percentage (%)                                                200° C.                                                                      300° C.                                                                           400° C.                                                                        500° C.                              ______________________________________                                        Example 1  0        0          0     50                                         Comparative 0 25 75 100                                                       example 1                                                                   ______________________________________                                    

The results as given in Table 1 show that the lead frame material ofExample 1 has good heat-resistance.

EXAMPLE 2

A lead frame material was produced in the same manner as in Example 1,except that the step 4 was not carried out. Thermomigration test wascarried out in the same manner as in Example 1, and the results aregiven in Table 2.

Comparative Example 2

A lead frame material was produced in the same manner as in ComparativeExample 1, except that the step 4 was not carried out. Thermomigrationtest was carried out in the same manner as in Example 1, and the resultsare given in Table 2.

                  TABLE 2                                                         ______________________________________                                                 Thermomigration percentage (%)                                                200° C.                                                                      300° C.                                                                           400° C.                                                                        500° C.                              ______________________________________                                        Example 2  0        0          0     50                                         Comparative 0 25 75 100                                                       example 2                                                                   ______________________________________                                    

The results as given in Table 2 show that the lead frame material ofExample 2 has good heat-resistance.

Industrial Applicability

The lead frame materials of the present invention are not only suitedfor producing fine lead areas but also excellent in heat-resistancebecause it has an etching stop layer made from a nickel-phosphorusalloy. The lead frame materials of the present invention are alsoexcellent in productivity because the etching stop layers thereof arenickel-phosphorus alloy layers which can be formed by electroplatingwithout requiring troublesome processes such as vapor deposition.

What is claimed is:
 1. A lead frame material, comprising:a copper orcopper alloy layer (A) of 35 to 300 μm thickness, wherein the copper orcopper alloy layer (A) is a copper alloy layer comprising an alloy ofcopper and at least one metal selected from the group consisting of Sn,Ni, Zn, P, Fe, Zr, Cr, Mg and Si; a nickel-phosphorus alloy layer (B) of1.6 to 10 μm thickness which is formed on the copper or copper alloylayer (A) and contains 0.3 to 1.0 wt % of phosphorus; and a copper layer(C) of 0.2 to 30 μm thickness which is formed on the nickel-phosphoruslayer (B).
 2. The lead frame material of claim 1, wherein thenickel-phosphorus alloy layer (B) contains 0.5 to 0.8 wt % ofphosphorus.
 3. The lead frame material of claim 1, wherein the copper orcopper alloy layer (A) has a surface roughness Ra of 0.1 to 2 μm.
 4. Thelead frame material of claim 1, wherein the nickel-phosphorus alloylayer (B) is formed by electroplating.
 5. The lead frame material ofclaim 1, wherein the copper layer (C) is 0.5 to 10 μm thick.
 6. A leadframe material, comprisinga copper or copper alloy layer (A) of 35 to300 μm thickness; and a nickel-phosphorus alloy layer (B) of 1.6 to 10μm thickness which is formed on the copper or copper alloy layer (A) andcontains 0.3 to 1.0 wt % of phosphorus.
 7. The lead frame material ofclaim 6, wherein the nickel-phosphorus alloy layer (B) contains 0.5 to0.8 wt % of phosphorus.
 8. The lead frame material of claim 6, whereinthe copper or copper alloy layer (A) is a copper alloy layer comprisingan alloy of copper and at least one metal selected from the groupconsisting of Sn, Ni, Zn, P, Fe, Zr, Cr, Mg and Si.
 9. The lead framematerial of claim 6, wherein the copper or copper alloy layer (A) has asurface roughness Ra of 0.1 to 2 μm.
 10. The lead frame material ofclaim 6, wherein the nickel-phosphorus alloy layer (B) is formed byelectroplating.
 11. The lead frame material of claim 6, which is atwo-layered lead frame material.